a. Field of the invention:
The present invention relates to a count error detecting device for count type measuring instruments with optical scales used in microscopes for measurement.
b. Description of the prior art:
A conventional circuit for count type measuring instruments with optical scales has comprised, as shown in, for example, FIG. 1, input terminals a and b for two signals A and B (FIG. 2) having a phase difference of 90.degree. each other to detect the directivity of measurement and perform interpolation which are derived from a detector D; a phase-shift circuit 1 composed of an inverting amplifier AMP inverting the signal A and voltage dividers R.sub.1, R.sub.2 and R.sub.3, R.sub.4 producing signals A+45.degree. and B+45.degree. which travel in phase by 45.degree. as compared with the signals A and B; comparators COMP.sub.1, COMP.sub.2, COMP.sub.3, COMP.sub.4 each of which is composed of a Schmitt circuit outputting a signal with constant amplitude when an input voltage is more than a preset value and which convert the signals A, A+45.degree., B, B+45.degree. into pulse signals A', A'+45.degree., B', B'+45.degree., respectively (FIG. 3); a coincidence circuit (exclusive OR circuit) EOR.sub.1 in which, when the signals A' and B' have different levels from each other, its output turns to a high (H) level and, when both the signals bear the "H" level or a low (L) level, the output turns to the "L" level; a coincidence circuit (exclusive OR circuit) EOR.sub.2 in which, when the signals A'+45.degree. and B'+45.degree. have different levels from each other, the output turns to the "H" level and, when both the signals bear the "H" level or the "L" level, the output turns to the "L" level; a pulse discriminator circuit 2 discrimating the directivity of measurement in comparison between the output signals (having a frequency twice that of the input signal each) of the coincidence circuits EOR.sub.1, EOR.sub.2 and supplying output signals corresponding to the discriminated result; and a counter circuit 3 counting up or counting down the number of output signals of the pulse discriminator 2. This circuit configuration is such that eight pulses (360.degree..div.90.degree..times.2=8) are to be counted with respect to each graduation of a scale not shown and thereby, for instance, if the scale is graduated each 4 .mu.m, measurement can be made with a unit of 0.5 .mu.m.
Also, the use of such a count type measuring instrument will cause defects that, as an example, when a stage on which an object to be measured is placed is moved at a high speed, a measurement frequency becomes extremely high, thus being in excess of followable frequencies of the detector or the subsequent circuits (namely, the pulse discriminator circuit 2 and the counter circuit 3), with the result that measurement errors are produced. The conventional circuit for count type measuring instruments has therefore added a count error detecting circuit, as depicted in FIGS. 1 and 4, comprising a leading edge detector circuit 4 detecting a leading edge of the output signal A' of the comparator COMP.sub.1 to output a signal .alpha.; an integrator circuit 5 initiating integration when the signal .alpha. is inputted to output a signal .beta.; a comparator COMP.sub.5 composed of the Schmitt circuit and outputting a signal .gamma. when the signal .beta. is inputted; and a one-shot timer circuit 6 configured to be actuated at a trailing edge of the signal .gamma. and to be reset at the leading edge and outputting an error signal .theta. when a preset time in seconds goes on without the timer reset after the timer actuation, and outputting an error signal through the utilization of pulse behaviors that, when a measurement frequency becomes higher than a preset value, the signal .beta. trails before its leading edge passes over the output level of the comparator.
However, a factor determining a detection speed limitation of the count type measuring instruments is largely composed of a followable frequency limitation of the detector and the error detecting circuit is adapted to previously detect the limitation. Specifically, although each amplitude of the two phase signals A, B issued from the detector D is reduced when the measurement frequency increases (FIG. 3), it must be detected as an error before being lower than the detection levels of the comparators COMP.sub.1, ..., COMP.sub.4. It is therefore necessary to raise the detection level of the comparator COMP.sub.5 to ensure a sufficient margin, so that the ability of the detector cannot sufficiently be displayed and generally measurement efficiency is decreased. In addition, since the followable frequency limitation of the detector depends on a coupling capacity thereof, which shows a wide variation, and also on an ambient temperature, its follows that a necessary margin is further increased and the measurement efficiency is more decreased.